In commonly assigned U.S. Pat. No. 4,584,592 issued on Apr. 22, 1986 in the names of Hsing C. Tuan and Malcolm J. Thompson entitled, "Marking Head For Fluid Jet Assisted Ion Projection Imaging Systems", there is disclosed a marking array for use in conjunction with the marking head of an ion projection printer of the type disclosed in commonly assigned U.S. Pat. No. 4,463,363 issued on July 31, 1984 in the names of Robert W. Gundlach and Richard L. Bergen, entitled, "Fluid Jet Assisted Ion Projection Printing". In that printer, an imaging charge is placed upon a moving receptor sheet, such as paper, by means of a linear array of closely spaced minute air streams. Charged particles, comprising ions of a single polarity (preferably positive), are generated in an ionization chamber of the marking head by a high voltage corona discharge and are then transported to and through the exit region of the marking head, where they are electrically controlled at each image pixel point, by an electrical potential applied to a modulating electrode. Selective control of the modulating electrodes in the array will enable spots of charge and absence of charge to be recorded on the receptor sheet for subsequent development.
A large area marking head for a page-width marking apparatus, such as a line printer or a copier, would typically measure about 8.5 inches wide. A high resolution marking array capable of printing 200 to 400 spots per inch would, therefore, include about 1700 to 3400 conductive metallic modulation electrodes. The entire array measuring on the order of 8.5 inches by 0.7 inches also would include a multiplexed addressing assembly comprising metallic address lines and data lines and amorphous silicon thin film active switching elements. All of these elements would be fabricated upon a single low cost substrate, such as glass.
During the operation of an ionographic marking apparatus, in addition to the ion flow from the corona source, there is also an outflow from the ionization chamber of gaseous material which is highly corrosive to the modulating electrodes and to any elements on the array which are not protected by suitable overcoating layers. Heretofore, the modulation electrodes have been fabricated of inexpensive electrically conductive materials which are compatible with standard thin film deposition techniques and which may be also used for conductive lines and for contacts with the active devices. Typically, this material has been aluminum. The ionization chamber effluents have a propensity to oxidize the exposed metallic modulation electrodes very rapidly, thereby lowering the operational lifetime of the marking array. It has been observed that the aluminum modulation electrodes oxidize continuously and rapidly, growing an insulating aluminum oxide layer which becomes increasingly unable to dissipate the ion charge built up thereon, so as to initially result in a deterioration of electrical modulation control characteristics, until finally, there will be a catastrophic electrical and mechanical failure as the electrodes are fully converted to the brittle insulating oxide which flakes off the substrate.
In two commonly assigned pending patent applications, there are disclosed ionographic marking heads having a resistance to corrosion, in this harsh ionographic environment, superior to that of the previously used aluminum electrodes. In U.S. Ser. No. 042,128 entitled "Modulation Electrodes Having Improved Corrosion Resistance" filed on Apr. 24, 1987 in the names of Nicholas K. Sheridon and Henry Sang, Jr. there is disclosed a marking head having modulation electrodes made of an aluminum and copper alloy which has an improved lifetime over unalloyed aluminum. It is believed that the improved lifetime arises from the copper "stuffing" the aluminum grain boundaries and preventing migration of oxygen through the material. This retards oxidization of the aluminum and slows its conversion to an insulating and embrittled material with the attendant deterioration of electrical and mechanical properties. In U.S. Ser. No. 042,131 entitled "Marking Array Having Improved Corrosion Resistance" filed on Apr. 24, 1987 in the names of Nicholas K. Sheridon and Henry Sang Jr. there is disclosed a marking head whose modulation electrodes are biased to a low level oxidation prevention potential, with similar beneficial results.
It is the primary object of the present invention to provide a marking array having an extended lifetime by incorporating modulation electrodes having surfaces substantially chemically neutral to the corrosive effluents of the ionographic process, so that the electrodes are not attacked and maintain their electrical and mechanical integrity.
Another object of the present invention is to provide modulation electrodes which will not build up an ion charge upon their surface.
Additionally, it is an object of this invention to provide modulation electrodes made of a materials which are inexpensive and which are compatible with the fabrication processes utilized in forming the marking head.